The nice thing about an oil warmer is that you're free to change the fragrance in it at any time simply by adding a different fragrance oil. The amount you use varies a bit depending on the model and whether the device uses a candle or electricity as the heat source. If you can't find an instruction page for your specific oil warmer, it's always better to start with just a little oil to avoid an overwhelming fragrance in the room.
Candle-Based Oil Warmers
One common oil warmer style uses a tealight candle as a heat source beneath a shallow bowl that holds water and fragrance oil. For this type of burner, fill the reservoir mostly full with water before adding any oils. Skipping the water and using only fragrance oil results in a burnt, goopy mess inside the reservoir.
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Essential oils and fragrance oils designed for this type of device often have a special top that drips out one drop at a time. Some fragrance oils are more potent than others, so start with about five or six drops of oil atop the water or use the amount recommended on the oil bottle. If your oil warmer is tiny, use fewer drops. Light the tealight candle beneath the bowl or reservoir. It takes five to 10 minutes before you'll notice much of a fragrance, as it takes a while to heat the water and the oil. For a stronger fragrance, add a couple more drops.
Over time, the water in the reservoir evaporates. Blow out the candle before the water completely evaporates or if you plan to leave the room for a while. Wait until the reservoir cools to near room temperature before adding more water to avoid splatters. Add more oil at any time when there's ample water in the bowl.
Electric Oil Warmers
One electric oil burner or warmer is completely different from the next, so it's always best to check the actual product instructions to determine the proper usage for that model. Some electric models do not use water at all; these generally do not have a bowl-style reservoir in them.
Plug in the burner and turn it on, allowing it to heat up for 15 minutes. Once it's warm, add up to 10 drops of oil in the larger opening on top and not through the small vent holes on the side. Don't move the unit while it's plugged in and on; allow it to cool down for 15 minutes before moving it. Some of these burners shut off automatically after 45 minutes or so.
Changing the Oil Fragrance
Both electric and candle-based oil warmers may be cleaned when they're cool. Unplug an electric oil warmer before cleaning it. Cleaning is a good idea any time you wish to use a different fragrance; otherwise, some of the old scent may come through the next time you use the burner. Wipe out the reservoir with a lint-free cloth dipped in white vinegar or rubbing alcohol to remove any oily residue, repeating as needed. For a candle-based oil burner that has old, gummy residue in the reservoir, pour some vinegar or rubbing alcohol directly into the reservoir and let it sit for 30 minutes or so before wiping it out.